1. Field of the Invention
This invention relates to apparatus and methods for surface analysis. Surface analysis is concerned with the determination of the elemental and chemical composition of the uppermost atomic layers of a material or component, together with the microstructural and physical properties in the vicinity of the surface and the actual surface configuration.
2. Description of the prior art
In recent years, surface sensitive analytical techniques have found increasing application in materials science and technology. Corrosion, catalysis and wear are just a few of the many surface/interface-specific materials phenomena for which bulk sampling analytical techniques, e.g. the X-ray microprobe analyser, are either ineffective or inappropriate. The most important requirement for a surface technique is the provision of an ultra-high vacuum environment in which a given surface condition can be established and maintained.
An array of surface sensitive techniques--AES, XPS, SIMS, PAS, SAM and SEM, as discussed below--are available to probe the physico/chemical characteristics of surfaces and interfaces. An outline of the nature and capabilities of each of these techniques follows.
Auger electron spectroscopy (AES) is a well established surface analytical technique which enables the chemical composition of solid surfaces to be qualitatively determined and, with calibration, quantitatively determined. Being a surface technique, AES requires the presentation or creation of a solid surface within an ultra-high vacuum, for example by fracture or ion beam erosion. This mandatory experimental environment keeps even the most reactive surfaces chemically stable for the duration of an analysis.
In AES, an electron beam is used to probe the surface and generate the Auger spectra. By using small electron beam diameters (approximately 1.mu.) and rastered beams, high resolution scanning Auger microscopy (SAM) maps of lateral elemental distributions can be generated. Sensitive to all elements except hydrogen and helium, AES samples only the top two or three layers of a surface. The addition of ion beam erosion enables elemental depth profiles into the bulk to be determined, but this necessarily involves destruction and removal of the surface layers in the region under study.
X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and scanning electron microscopy (SEM) also each involve the bombardment of a surface located in a high or ultra-high vacuum environment with either a photon flux, an ion beam, or an electron beam, as appropriate, so that electrons or ions are emitted.
The physical and chemical characteristics of the surface of the sample under examination may then be established from analysis of the energies and masses of the emitted particles, which are characteristic of their origin. Three-dimensional data as to the distribution of the elements may be achieved by a combination of operations, the ion or electron beam being first of all scanned or rastered over the region of the surface to establish the two-dimensional distribution, followed by controlled sputtering of the material by an ion beam to provide elemental and chemical information on the variation in the composition of the sample with depth.
Photoacoustic or thermal spectroscopy (PAS) makes use of a phenomenon which arises when intensity-modulated electromagnetic radiation, e.g. light, is absorbed by a sample of material, thus exciting energy levels within it. De-excitation of these levels occurs for the most part by means of a non-radiative or heat-producing process. Hence the absorption of intensity-modulated electromagnetic radiation at any point in the sample results in a periodic localised heating of the material. Photoacoustic studies performed on gaseous and powdered material generally involve the use of a microphone, which detects pressure fluctuations or acoustic waves arising out of the periodic pressure rise either in the gas under study or in gas heated by proximity to the sample of powdered material under test, as appropriate.
Photoacoustic spectroscopy may also provide a non-destructive method for evaluating the optical and thermal properties of opaque and other solids. When energy from a modulated light or particle beam is absorbed by a solid sample, localised heating occurs which generates an elastic stress that propagates through the sample as an acoustic signal at the modulation frequency. This signal is a complex quantity whose magnitude and phase depend on the absorption and thermal characteristics of the sample. In circumstances where the signal does not saturate, it is related to the absorption coefficient so that the photoacoustic spectrum that results corresponds to the conventional absorption spectrum of the material of the sample. A particular advantage of PAS over other techniques is that because the detected signal depends only on light that is absorbed, scattered or transmitted light is not normally a problem.
Thus, PAS may be used to great advantage in the study of absorbed and chemisorbed molecules on surfaces, provided that the substrate itself is either non-absorbing or highly reflecting over the wavelength range. Infra-red absorption is particularly useful because the fine-structure associated with the infra-red spectra provides a sensitive probe for chemical identification and quantification. A tunable CO.sub.2 laser operating between 9.mu. and 11.mu. at power levels of 0.1-1 W has been used, for instance, to detect submonolayer coverage of absorbed species on silver surfaces.
Detection of the photoacoustic signal in solids may be achieved by placing a piezoelectric transducer in intimate contact with the sample; these devices are sensitive to temperature changes of 10.sup.-7 K and, unlike gas-microphone detection methodologies, may be made UHV compatible.
Each of the various surface sensitive analysis techniques discussed above has individual special features which render it particularly appropriate to some specific aspect of surface analysis. For example, SIMS facilitates detection of elements present in very minute quantities, while others of the techniques are preferred for picturing topographical details at high resolution or for providing chemical information as to the composition of samples sensitive to damage by particular forms of beam bombardment. A common approach nowadays is to combine a number of these techniques into a simultaneous experimental study. This procedure helps to unravel synergistic property relationships and to deconvolute artefacts arising from individual techniques.
Many surface studies are, however, hindered by the absence of a complementary technique for providing microstructural and other physical properties of the sub-surface without destroying the surface or interface in question, as occurs with sputtering or erosion. Such information, important in its own right, would provide, together with the surface data, a more complete characterisation of surface/interface behaviour.
Pulsed thermal microscopy may be used for surface and sub-surface imaging of the physical integrity of solids. In one version, a technique generally referred to as thermal wave microscopy (TWM), which is a recent development of the older technique of PAS, microscopic and macroscopic features on and beneath the surface of a sample may be detected and imaged non-destructively. The modulated thermal disturbance that occurs within the absorbing volume of a solid sample in PAS and other techniques propagates outwards as a critically damped evanescent thermal wave whose range is of the order of the thermal diffusion length. By rastering a focussed energy probe (laser, electron, ion) across the sample surface, thermal wave images at selected depths within the sample may be obtained. Image contrast results from reflection and scattering interactions of the thermal waves with mechanical and crystallographic artefacts within the image field. Thus, thermal wave microscopy may be used to locate micron scale flaws, crystallographic grain heterostructure and other thermally sensitive features of solid surfaces and subsurfaces.
The resolution, in some circumstances, depends on the thermal wavelength, which in turn is determined by the modulation frequency; the use of megahertz frequencies, with appropriate probe diameters and detector response times, optimises the spatial resolution in these cases to better than 1.mu.. While adjustment of the signal phase provides selective examination of thermal features at various depths within the image field, full-range depth profiling leading to three-dimensional image reconstruction requires the image field depth itself to be adjusted by varying the modulation frequency.
It may be noted that the term "thermal wave microscopy", as used herein and as explained in the foregoing paragraph, represents a current theory which satisfactorily explains the phenomena observed when a pulsed energy input is applied to a sample of a material. Thus while the phenomena in question are detectable and capable of analysis on the assumption that the pulsed energy input results in the establishment of thermal waves within the material of the sample, it is not necessarily the case that the effect observed may not also be explained, either for all or some of said phenomena, in terms of a different form of behaviour of the material under localised pulsed excitation by an energy input. The difficulties in determining and explaining the behaviour of materials when dealing with very thin layers having thicknesses of the order of only a small number of atoms are well known to those skilled in this art, and the necessity to proceed on the basis of theoretical explanations of the behavioural phenomena which are not necessarily definitive or correct in all experimental circumstances is also well known. Thus the term "thermal wave microscopy" is to be understood in the present text in the context of the foregoing remarks and as representing, in essence, a conceptual explanation of thermal microscopy in which a pulsed energy input is applied to a sample.